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  • Cardiac arrhythmias can be classified by site of origin:

  • - Sinus rhythms originate from the sinoatrial node, or SA node

  • - Atrial rhythms originate from the atria - Ventricular rhythms originate from the ventricles.

  • Sinus rhythm is the normal rhythm of the heart set by its natural pacemaker in the SA node.

  • In a healthy heart, the SA node fires 60 to 100 times per minute resulting in the normal

  • heart rate of 60 to 100 beats per minute.

  • The most common variations of sinus rhythm include:

  • - Sinus bradycardia: when the SA node fires less than 60 times per minute resulting in

  • a slower heart rate of less than 60 beats per minute.

  • and - Sinus tachycardia: when the SA node fires

  • more than 100 times per minute generating a faster heart rate of greater than 100 beats

  • per minute.

  • Sinus bradycardia and sinus tachycardia may be normal or clinical depending on the underlying

  • cause.

  • For example, sinus bradycardia is considered normal during sleep and sinus tachycardia

  • may be normal during physical exercises.

  • Cardiac arrhythmias that originate from other parts of the atria are always clinical.

  • The most common include: atrial flutter, atrial fibrillation and AV nodal re-entrant tachycardia.

  • These are forms of supraventricular tachycardia or SVT.

  • Atrial flutter or A-flutter is caused by an electrical impulse that travels around in

  • a localized self-perpetuating loop, most commonly located in the right atrium.

  • This is called a re-entrant pathway.

  • For each cycle around the loop, there is one contraction of the atria.

  • The atrial rate is regular and rapid - between 250 and 400 beats per minute.

  • Ventricular rate, or heart rate, however, is slower, thanks to the refractory properties

  • of the AV node.

  • The AV node blocks part of atrial impulses from reaching the ventricles.

  • In this example, only one out of every three atrial impulses makes its way to the ventricles.

  • The ventricular rate is therefore 3 times slower than the atrial rate.

  • This is an example of a “3 to 1 heart block”.

  • Ventricular rate in A-flutter is usually regular, but it can also be irregular.

  • On an ECG atrial flutter is characterized by absence of normal P wave.

  • Instead, flutter waves, or f-waves are present in saw-tooth patterns.

  • Atrial fibrillation is caused by multiple electrical impulses that are initiated randomly

  • from many ectopic sites in and around the atria, commonly near the roots of pulmonary

  • veins.

  • These un-synchronized, chaotic electrical signals cause the atria to quiver or fibrillate

  • rather than contract.

  • The atrial rate during atrial fibrillation can be extremely high, but most of the electrical

  • impulses do not pass through the AV node to the ventricles, again, thanks to the refractory

  • properties of the cells of the AV node.

  • Those do come through are irregular.

  • Ventricular rate or heart rate is therefore irregular and can range from slow - less than

  • 60 - to rapid -more than 100 - beats per minute.

  • On an ECG, atrial fibrillation is characterized by absence of P-waves and irregular narrow

  • QRS complexes.

  • The baseline may appear undulating or totally flat depending on the number of ectopic sites

  • in the atria.

  • In general, larger number of ectopic sites results in flatter baseline.

  • AV nodal re-entrant tachycardia or AVNRT is caused by a small re-entrant pathway that

  • involves directly the AV node.

  • Every time the impulse passes through the AV node, it is transmitted down to the ventricles.

  • The atrial rate and ventricular rate are therefore identical.

  • Heart rate is regular and fast, ranging from 150 to 250 beats per minute.

  • Ventricular rhythms are the most dangerous.

  • In fact, they are called lethal rhythms.

  • Ventricular tachycardia or V-tach is most commonly caused by a single strong firing

  • site or circuit in one of the ventricles.

  • It usually occurs in people with structural heart problems such as scarring from a previous

  • heart attack or abnormalities in heart muscles.

  • Impulses starting in the ventricles produce ventricular premature beats that are regular

  • and fast, ranging from 100 to 250 beats per minute.

  • On an ECG V-tach is characterized by wide and bizarre looking QRS complexes.

  • P wave is absent.

  • V-tach may occur in short episodes of less than 30 seconds and cause no or few symptoms.

  • Sustained v-tach lasting for more than 30 seconds requires immediate treatment to prevent

  • cardiac arrest.

  • Ventricular tachycardia may also progress into ventricular fibrillation.

  • Ventricular fibrillation or v-fib is caused by multiple weak ectopic sites in the ventricles.

  • These un-synchronized, chaotic electrical signals cause the ventricles to quiver or

  • fibrillate rather than contract.

  • The heart pumps little or no blood.

  • V-fib can quickly lead to cardiac arrest.

  • V-fib ECG is characterized by irregular random waveforms of varying amplitude, with no identifiable

  • P wave, QRS complex or T wave.

  • Amplitude decreases with time, from initial coarse v-fib to fine v-fib and ultimately

  • to flatline.

Cardiac arrhythmias can be classified by site of origin:

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Cardiac Arrhythmias

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    alex posted on 2017/10/14
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